HVDC (high-voltage, direct current) electric power transmission systems use direct current for the bulk transmission of electrical power. In particular for the transmission over long distances, HVDC systems suffer lower electric losses than alternating current (AC) systems.
Electric energy generated in e.g. large wind farms is typically collected in AC offshore stations and transmitted to HVDC offshore stations where the conversion from AC to DC is performed.
The platform of these stations comprises a converter building in which the respective electrical active parts are arranged. The size and weight of the converter building and of the platform, respectively, are related to the power capacity of the station.
For example, the platform of a 1000 MW HVDC station has a weight of roughly 10,000 tons. The volume of the respective converter building is typically in the order of several 10,000 m3. In a conventional Dolwin converter, for example, one valve hall alone typically has a size in the order of about 10,000 m3.
At present such platforms must be large in size in order to handle the required amounts of electric power. High voltages are needed to keep the currents, electric losses and conductor or cable cross-sections sufficiently low. High voltages need long insulation distances and thus require a lot of space, if air is used as the insulation medium. Air, while having the advantage of being available ubiquitously at no cost, has a rather low insulation performance. Furthermore, installation of a platform comprising a converter building of this weight and size may require cranes with extremely high capacity which are rarely available. Thus, conventional HVDC offshore stations suffer from their enormous size and weight.
A reduction in size and weight of the platform and the converter building is highly desirable to simplify the complexity of installation, operation, maintenance, and deconstruction of conventional stations.